1. Field of the Invention
This invention relates to a X-ray tube such as a penetrating type X-ray tube, especially to an X-ray tube of this kind where exfoliation at the interface of a target film from which X rays generate, formed on the inner surface of an X-ray transmission window plate being a part of its evacuated envelope is prevented from occurring and to a manufacturing method thereof.
2. Description of the Related Art
An X-ray tube has a structure in which X rays are emanated by an electron beam collided with an anode target. The X-ray tube is used in various fields such as industrial use e.g. medical diagnosis, nondestructive testing or material analysis. Various kinds of X-ray tubes are in use according to these demands. Among them, there is a transmission emanating type X-ray tube that offers a minute focal spot, i.e. micro focus X-ray source.
One of utilization of the micro focus transmission emanating type X-ray tube is an X-ray fluoroscopic magnified image pickup device for semiconductor integrated circuit boards or other objects. As shown in FIG. 7, X-ray tube 12 is accommodated in device case 11 which shields X rays. Object 13 such as the semiconductor integrated circuit board is set at the position apart from X-ray focal point S of X-ray tube 12 by the distance La. Furthermore at the position apart from object 13 by the distance Lb, the sensor surface of X-ray area sensor such as an X-ray image tube or a solid state X-ray sensor is positioned facing thereto.
An operating voltage which is controlled from outside is to be supplied to X-ray tube 12 by means of power source 15 contained in case 11. An X-ray image signal derived from X-ray image signal output section 16 of X-ray area sensor 14 is transmitted to monitor 17 having a image processing device so as to display an X-ray fluoroscopic magnified image of object 13 on image display section 18.
Magnification M of an X-ray imaging of an object is represented approximately by M=(La+Lb)/La. Since both the distances are set to be La((Lb, the smaller the distance La becomes, the larger the magnification M increases. It is also self-evident that the smaller the size of focus S which is the origin of X rays in the X-ray tube is, the clear the X-ray fluoroscopic magnified image becomes.
Therefore, it is desirable that the focus S of the X-ray tube, i.e. the X-ray emanating target section should be located as close as possible to object 13, in order to make distance La as small as possible. For this purpose, utilization of a micro focus transmission emanating type X-ray tube in which an X-ray emanating target is at the utmost tip of the tube is suitable.
As shown in FIG. 8, such X-ray tube 12 has X-ray transmission window 22 permeable to X rays, provided vacuum-tightly at one of the tips of the metallic hollow cylinder of evacuated envelope 21. Transmission window 22 is usually made of a material highly permeable to X rays such as beryllium (Be). On the surface of the evacuated side of transmission window 22, anode target film 23 of tungsten (W) etc. is directly stuck, as the main part is shown magnified. Inside the glass portion of the other side of the evacuated envelope, cathode 24 emitting an electron beam is mounted and electron gun 25 comprising the cathode and a plurality of grid electrodes for an electron lens is accommodated.
In the above mentioned structure, electron beam e emitted out of the cathode and passing through electron lens 25 is designed to make point focus S at anode target film 23. Then, X rays generated at the anode target film are emanated out as they are, via transmission window 22. The emanated X rays represented by mark X are used for X-ray imaging.
The apparatus or the X-ray tube like this is disclosed, for example, in U.S. Pat. No. 5,077,771, Japanese Patents No. 2,713,860, No. 2,634,369, Japanese Patent Publication No. Hei/7-50594, Japanese Patent Disclosure No. Hei/9-171788, Japanese Utility Model Publication No. Shou/52-56778, and Japanese Utility Model Disclosure No. Shou/54-163885.
The optimum thickness of the tungsten film constituting the anode target of the transmission emanating type X-ray tube depends on the voltage supplied to the tube. For instance, in the case of industrial X-ray tubes, the voltage supplied to the X-ray tube is generally in the range from several tens kV to one hundred and several tens kV. In such a case, the optimum thickness of the tungsten film constituting the anode target is in the range from several xcexcm to ten and several xcexcm.
The structure where a tungsten film constituting the anode target is directly stuck to the inner surface of the beryllium X-ray transmission window is apt to generate an interfacial exfoliation between tungsten and beryllium, and to result in unstable state, under the influence of the remaining stress in the film generated while the tungsten film is being formed or of the difference of thermal expansion coefficient between tungsten and beryllium constituting the transmission window.
Especially, for a micro focus penetrating-type X-ray tube, the interfacial exfoliation is liable to take place at the micro focus part, because an electron beam having, for example, a focus of substantially circular configuration of several tens xcexcm or less in diameter impinges on the tungsten film. If the interfacial exfoliation takes place, it is thought that melting of the tungsten film or spattering of exfoliated material caused by local irradiation of the electron beam may result in serious damage of the X-ray tube.
The object of this invention is to solve the above mentioned shortcomings and to provide a highly reliable penetrating type X-ray tube and a manufacturing method thereof, preventing interfacial exfoliation between a transmission window and a target film before it happens.
The penetrating type X-ray tube according to the present invention has a X-ray transmission window plate of beryllium stuck vacuum-tightly to a portion of an evacuated envelope, a target film of tungsten or of an alloy mainly constituted of tungsten which is provided on the evacuated side of the window plate and emanates X rays, and at least one intermediate film of at least one metal element such as copper or of a material principally constituted of this metal element intervening closely between the window plate and the target film.
The manufacturing method according to the present invention is characterized in that at least one intermediate film of at least one metal element selected from copper, chromium, iron, nickel, etc. or of a material principally constituted of this metal element and an X-ray generating target film are formed on the inner surface of the X-ray transmission window of beryllium and on the intermediate film respectively by a physical vapor deposition method such as spattering.